A tensioning device, such as a hydraulic tensioner, is used as a control device for a power transmission chain, or similar power transmission device, as the chain travels between a plurality of sprockets. In this device, the chain transmits power from a driving shaft to a driven shaft, so that part of the chain is slack and part of the chain is tight. Generally, it is important to impart and maintain a certain degree of tension in the chain to prevent noise, slippage, or the unmeshing of teeth in the case of a toothed chain.
Prevention of such slippage is particularly important in the case of a chain driven camshaft in an internal combustion engine because jumping of teeth will throw off the camshaft timing, possibly causing damage or rendering the engine inoperative. However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension.
For instance, wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tensions. Reverse rotation of an engine, occurring for example in stopping or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism is desired to remove excessive tensioning forces on the tight side of the chain and to ensure the necessary tension on the slack side of the chain.
Hydraulic tensioners are a common method of maintaining proper chain tension. In general, these mechanisms employ a lever arm that pushes against the chain on the slack side of the power transmission system. This lever arm must push toward the chain, tightening the chain, when the chain is slack, and must be very rigid when the chain tightens.
To accomplish this result, a hydraulic tensioner typically comprises a rod or cylinder as a piston, which is biased in the direction of the chain by a tensioner spring. The piston is housed within a cylindrical housing, having an interior space which is open at the end facing the chain and closed at the other end. The interior space of the housing contains a pressure chamber in connection with a reservoir or exterior source of hydraulic fluid. The pressure chamber is typically formed between the housing and the piston, and it expands or contracts when the piston moves within the housing.
Typically, valves are employed to regulate the flow of fluid into and out of pressure chamber. For instance, an inlet check valve typically includes a ball-check valve that opens to permit fluid flow in to the pressure chamber when the pressure inside the chamber has decreased as a result of outward movement of the piston. When the pressure in the pressure chamber is high, the inlet check valve closes, preventing fluid from exiting the pressure chamber, which in turn prevents the piston chamber from contracting, which in turn prevents the piston from retracting, achieving a so-called "no-return" function.
Many tensioners also employ a pressure relief mechanism which allows fluid to exit the pressure chamber when the pressure in the chamber is high, thus allowing the piston to retract in response to rapid increases in chain tension. In some tensioners, the pressure relief mechanism is a spring biased check valve, which opens when the pressure in the pressure chamber becomes high. Some tensioners may employ a valve which performs both the inlet check function as well as the pressure relief function. Other mechanisms employ a restricted path through which fluid may exit the fluid chamber, such that the volume of flow exiting the fluid chamber is minimal unless the pressure in the fluid chamber is great. For instance, a restricted path may be provided through the clearance between the piston and bore, through a vent tube in the protruding end of the piston, or through a vent member between the fluid chamber and the fluid reservoir.
A number of challenges exist in the design of hydraulic tensioners. One general design problem is the high cost and difficulty of manufacture and assembly. Traditionally, hydraulic tensioners have been constructed of cast iron housing bodies. The cast metal components provide the required close fit between the housing and the piston, and provide for strength and durability of the tensioner. However, this type of construction is expensive and difficult to manufacture. A need exists for a lower cost hydraulic tensioner which is easier to manufacture and assemble.
One example of a tensioner design directed to reduced cost is described in Ojima et al., U.S. Pat. No. 5,037,357. Ojima et al. disclose a spring loaded tensioner including a body having a bearing surface, a first spring seated against the bearing surface and biasing a piston in a protruding direction. A second spring functions as a damper allowing the piston to retract in response to increasing tension in the belt or chain. The body may be made of sheet metal, allowing for low cost manufacturing. The disadvantages of this design include the reliance on springs to provide the "no return" and pressure relief functions. As a result, this design does not provide the advantages in performance provided by a hydraulic tensioner.
Another tensioner known in the art employs a metal insert positioned within the bore of the housing body. The metal insert has cylindrical body and a solid bottom seated in the end of the bore. The fluid chamber is formed between the metal insert and the piston. Because the fluid chamber is formed with the metal insert, rather than the bore of the housing, the housing may be made of a less expensive material such as plastic. However, the metal insert may be difficult and expensive to manufacture and assemble. In particular, a cup-shaped insert may be difficult and expensive to manufacture. Moreover, this tensioner lacks a spring to bias the piston outward, and instead the tensioner relies on oil pressure in the fluid chamber to bias the piston outward.
Another problem in the design of tensioners is excessive retraction of the piston during engine start up. Such retraction may produce undesirable noise in the system, or may allow the chain to slip or skip a tooth. One cause of such retraction is the leakage of oil from either the fluid chamber or the oil reservoir while the engine is off. For instance, fluid may leak from the fluid chamber through a clearance between the piston and the bore. Fluid may also leak from the oil reservoir, particularly in the case of an oil reservoir positioned in a cast iron housing. Such leakage may be accompanied by the introduction of air into the fluid chamber. Because air is more compressible than fluid, the presence of air in the fluid chamber allows significantly greater piston retraction, and reduced tensioner performance.
Excessive piston retraction during engine start up may also be caused by force applied by the chain while the engine is off. For instance, if the vehicle is left on a hill, the rotational force on the wheels may cause increased chain tension in the engine. The resulting increase in chain tension may cause leak down of the piston, and poor tensioner performance during engine start-up. Thus, a need exists for a tensioner design which avoids excessive piston retraction during engine start-up.
The problems of leakage of fluid from the fluid chamber and undesired piston retraction also affect the means available for minimizing the cost to manufacture and assemble the tensioner. A poor fit between the piston and the bore of the housing allows greater leakage of fluid from the fluid chamber. Further, it is difficult and expensive to maintain the required close manufacturing tolerances of the bore and piston to avoid excessive leakage. Typically, the tensioner body is formed of cast iron or steel, with the bore machined out for the piston and check valve assembly. The disadvantages of this system include the expense of the casting system, which may require specialized manufacturing machines. In addition, the dimensional accuracy of the boring machine is limited.
Accordingly, it is an object of the present invention to provide a hydraulic tensioner which can be inexpensively manufactured and assembled. It is another object of the present invention to provide a hydraulic tensioner having improved response to fluctuations in chain tension. It is a further object of this invention to provide a hydraulic tensioner having improved performance at engine start-up. It is yet another object of this invention to provide a hydraulic tensioner which is less susceptible to leakage from its fluid chamber.
It is another object of this invention to provide a method of producing a less expensive and more effective hydraulic tensioner.